(1) Field of the Invention
The present invention relates to organic EL (electro-luminescent) elements.
(2) Description of the Related Art
With rapid advancement of recent data processing units, considerable attention has been paid to EL elements to meet the increasing need for flat-display elements that consume less electricity and require less space compared with a CRT (cathode-ray-tube).
The EL elements include non-organic EL elements and organic EL elements. The non-organic EL elements emit light, or luminesce, when fully accelerated electrons collide with luminous materials to excite the same. On the other hand, the organic EL elements luminesce when electrons and holes, which are injected from an electron injecting electrode (cathode) and a hole injecting electrode (anode) respectively, re-combine in a luminous layer. The organic EL elements are advantageous over the non-organic EL elements in that they demand a lower driving voltage: the former demand about 5-20 V while the latter demand about 100-200 V. Moreover, the organic EL elements can luminesce three primary colors when they are made of adequate fluorescent materials, and thus are expected to enable a full-color display unit.
For further understanding, the structure of the organic EL elements will be explained more in detail.
The typical organic EL elements are either a three-layer or two-layer structure comprising an organic luminous layer and an organic carrier (electrons or holes) transport layer.
The three-layer structure known as a DH (Double Hetero) structure comprises an organic hole transport layer, an organic luminous layer, and an organic electron transport layer. A hole injecting electrode (anode) is placed on a glass substrate, on which the organic hole transport layer, organic luminous layer, organic electron transport layer and an electron injecting electrode (cathode) are sequentially layered one on top of another.
The two-layer structure includes structures known as an SH-A (Single Hetero-A) structure and an SH-B (Single Hetero-B) structure.
The SH-A structure excludes the organic electron transport layer, comprising the organic hole transport layer and organic luminous layer. The hole injecting electrode is placed on a glass substrate, on which the organic hole transport layer, organic luminous layer, and electron injecting electrode are sequentially layered one on top of another.
The SH-B structure excludes the organic hole transport layer, comprising the organic electron transport layer and organic luminous layer. The hole injecting electrode is placed on a glass substrate, on which the organic luminous layer, organic electron transport layer, and electron injecting electrode are sequentially layered one on top of another.
The hole injecting electrode is made of electrode materials having a large work function such as Au (gold) or ITO (In-Sn oxide); the work function of a metal means the energy supplied to free electrons to enable them to escape from the metal. Conversely the electron injecting electrode is made of electrode materials having a small work function, such as Mg (magnesium).
The organic hole transport layer is made of an organic material of p-type, while the organic electron transport layer is made of an organic material of n-type. The organic luminous layer is made of an organic material of n-type, p-type, and almost neutral in the SH-A structure, SH-B structure, and DH structure, respectively.
In any structure, the organic EL elements luminesce when the holes injected from the hole injecting electrode and the electrons injected from the electron injecting electrode recombine within the luminous layer at the boundary with the hole transport layer or electron transport layer.
Although the organic EL elements are advantageous over the non-organic EL elements, they face some problems to be solved. In particular, durability stands in the way of practical applications.
The luminosity of conventional organic EL elements degrades rapidly; a luminosity half-life is only tens hours when driven at a stable current. Causes of the degradation remain unrevealed; however, carriers accumulating at the boundary between the organic luminous layer and organic carrier transport layer are assumed to cause such degradation.
A report presented by Mori et al at "The 50th Applied Physics Lecture" at Nagoya University, Fall, 1989, 29p-ZP-7, reveals that the luminosity degradation of the organic EL elements subject to continuous luminescing can be recovered to some degree by applying a reverse bias voltage. This implies that the luminosity degradation is caused by the accumulated carriers at the boundary between the organic carrier transport layer and organic luminous layer.
Heat generated while the organic EL elements is driven is also assumed to cause the degradation. The heat causes molecules in each layer to crystalize, thus changing the structure between molecules.